The basic objective of the proposed work is to elucidate the molecular mechanisms responsible for signal generation and amplification in photoreceptors. One possible mechanism of primary excitation in rod cells involves the release of Calcium ion from the lumen of the disc membranes, and the light dependent permeability of native and reconstituted disc membranes will be measured in isolated systems. New methods are proposed to measure transmembrane potentials, and these will be applied to the photoreceptor membranes in conjunction with the permeability studies. The light activated phosphorylation reaction of rhodopsin will be investigated, and the influence of phosphorylation on membrane permeabilities described. Spin labeling methods will be used to investigate the dependence of rhodopsin conformation on light and phosphorylation. Finally, freeze-fracture studies on squid rhabdomes, and chemical characterization of the squid rhodopsin will be initiated.